During the collection and granulating of scrap plastic material metal components of the processing machinery itself or of the mold or miscellaneous small pieces of metal may fall into the auger feed trough and be conveyed into the granulating chamber where it can cause damage to the cutting knives. The pieces of metal, including any metal from the damaged knives, contaminates the reground plastic which is normally fed back to the principal plastic processing equipment where the metal contaminants could cause damage to the plastic processing equipment creating loss of production and defective molded parts.
It has been proposed that metal detectors for use in auger feed granulators magnetically detected the presence of metallic contaminants in the scrap plastic material. These magnetic detectors are somewhat limited since they are sensitive only to ferrous materials that exhibit magnetic properties. Thus, non-ferrous objects would elude such a detector thereby damaging the cutting knives and contaminating the granulated plastic. Other detectors that electrically detected the presence of metal contaminants are cumbersome, complicated and and unduly costly to manufacture.